Ion Mobility Spectrometers (IMS) identify trace constituents of a gas mixture by ionizing them, introducing a number of the resulting ions into a space to which an electric field is applied, and measuring the time taken for the ions to traverse the length of this drift space under the influence of the field. At the end of the drift space, the ions strike a collector electrode and produce current pulses, and the time history of the collector current, showing a series of pulses corresponding to the arrival of ions of different types, is known as a plasmagram. The length of time, t.sub.d, for an ion to traverse the drift space and reach the collector electrode is known as its drift time, and depends on the size and mass of the ion and the charge it carries, on the length of the drift space, and on the field strength, temperature and gas pressure therein. To a first approximation, this dependence can be described by the relation EQU t.sub.d =(1/K.sub.0)*(L/E)*(273.5/T)*P/760) (1)
where T is the gas temperature (degrees Kelvin), P is the pressure (Torr), L is the length of the drift space, E is the electric field (assumed uniform over L), and K.sub.0, commonly called the reduced mobility, is a constant characteristic of the particular ion. Because of the statistical nature of the collision process, and the existence of a radial component to the electric field, there will actually be a narrow range of drift times for each species of ion. That is, even if all the ions of a given species enter the drift space simultaneously, they will reach the collector at different times, and produce a current pulse of finite width. In a practical instrument, the plasmagram peak is the convolution of this pulse with the shape of the voltage pulse used to gate ions from the ionization region into the drift region. The widths of the plasmagram peaks set a limit to the resolution of the IMS, that is, the ability to distinguish between ions with similar values of K.sub.0.
The operation of an IMS is strongly affected, in many ways, by the presence of water vapour in the gas in the ionization and drift regions. In particular, neutral water molecules will form clusters with many types of analyte ion, with the number of water molecules clustered with each ion depending on the nature of the ion, the gas temperature, and the concentration of water molecules. These clusters are larger than the original ion, and thus have lower values of K.sub.0. Clusters may grow or shrink during their drift time, and thus exhibit K.sub.0 values intermediate between those for clusters with integer numbers of water molecules. The net effect is that, if the concentration of water increases, the drift time for a ion which forms clusters will increase and the peak will widen, which may cause the ion to be miss-classified, or to overlap and hide another peak. For this reason, most IMS instruments continuously purge the ionization and drift spaces with a gas flow which has been carefully dried to a water content of a few parts in 10.sup.5. This is an extremely low level of water vapour, and an operator needs to know if the drying system is not meeting the permissable maximum humidity level. Accordingly, there is a need for a device and method to monitor the performance of the drying system. This is especially true for instruments where the drift gas is atmospheric air dried by a consumable desiccant. Even with scheduled preventive maintenance, and the use of indicating desiccants, many factors, ranging from operator inattention to unusually high ambient humidity, may result in the IMS being operated under non-optimum conditions, with consequent loss of sensitivity and increase in false-alarm rate. Also, the humidity levels involved are below the range of conventional commercial moisture sensors.
The prior art proposes several solutions to the problem of instability in the IMS drift times of certain species due to variable formation of clusters with water. U.S. Pat. No. 5,405,781 teaches the use of a filter to remove water from the gas circulated through the IMS, and also the advisability, in IMS systems which use calibrants to compensate for temperature and like variables, of selecting, as calibrants, materials which have a minimal tendency to form clusters with water. U.S. Pat. No. 5,405,781 teaches the use of a supplementary thermal gas drying system to prolong the useful life of an absorption filter in an IMS. However, neither this patent nor other prior art reveal any means of monitoring the water vapour concentration in an IMS so as to be assured that it is within acceptable limits for the application.